Laser scanning systems typically utilize galvanometer motors to change the angle of scanning mirrors. Usually the X and y direction is scanned by separate motors. In many applications such as marking or cutting, a laser beam is scanned on a workpiece. To achieve a high power density, the laser beam is usually focused on this workpiece. Specialized "flat field" lens have been developed to achieve a good focus on a flat surface workpiece even at a high transmission angle. However, some applications require that the laser beam can be independently focused to accommodate a contoured surface. Normally, this focusing is accomplished by translating one or more lenses in an optical system to achieve a variable focal length. Unfortunately, high powered CO2 lasers can cause a thermal distortion in lenses which increases the focused spot size. Therefore, it is desirable to utilize all reflective optical components for high power CO2 laser applications. U.S. Pat. No. 5,561,544 titled "Laser Scanning System With Reflecting Optics", issued Oct. 1, 1996 by the same inventor as this application, represents a prior approach to solving this problem. That patent is included herein in its entirety by reference.
In my '544 patent, an off-axis elliptical mirror is used to focus the laser beam on the workpiece. An off-axis elliptical mirror actually has two focus points. In this case, one focus point is a long distance from the ellipse and is used to focus on the workpiece. The other focus point is near to the ellipse and is internal to the optical system. To separate the incident and reflected beams associated with this off-axis elliptical mirror, there is typically a 45.degree. angle between rays coming from the internal focus point and leading to the distant focus point. This off-axis geometry introduces less desirable characteristics such as the need for a special focus adjustment technique which does not introduce a translation to the external or distant focus point. However, the most significant drawback is the fact that the 45.degree. off-axis ellipse introduces astigmatism to the laser beam when there is a substantial change in the focal length. Another way of saying this is that the adjustment range of focal lengths to the external focus is smaller than desired. The present invention overcomes this focal adjustment problem and also utilizes fewer mirrors than in my '544 patent.